Work is force times distance. How tough your car (or anything else) is depends on:
1) How much force it takes to flex it -- the spring constant
2) How far it will flex before it breaks
Multiply the two together (or specifically, 1/2 kd^2 where d is how far it will bend before it breaks, k is the spring constant, and 1/2 kd is the average force you're applying), and you get how much energy it takes to break it.
*Everything* flexes. Rigid things just flex less for the same force.
Things which break when flexed just a little bit are called brittle.
1) How much force it takes to flex it -- the spring constant
2) How far it will flex before it breaks
Multiply the two together (or specifically, 1/2 kd^2 where d is how far it will bend before it breaks, k is the spring constant, and 1/2 kd is the average force you're applying), and you get how much energy it takes to break it.
*Everything* flexes. Rigid things just flex less for the same force.
Things which break when flexed just a little bit are called brittle.